Helical thermal switch



June 4, 1968 R- D. GRAYSON HELICAL THERMAL SWITCH 2 Sheets-Sheet 1 FiledSept. 22, 1965 Alva/r02.

m m m 0 D m n m R Arr-02 v R. D. GRAYSON HELICAL THERMAL SWITCH June 4,1968 2 Sheets-Sheet 2 Filed Sept. 22. 1965 United States Patent3,387,246 HELICAL THERMAL SWITCH Richard D. Grayson, Arcadia, Califl,assignor to International Telephone and Telegraph Corporation, acorporation of Delaware Filed Sept. 22, 1965, Ser. No. 489,220 Claims.(Cl. 337-372) ABSTRACT OF THE DISCLOSURE A thermally responsiveover-temperature switch includes an elongated helical bimetallic elementwhich surrounds one end of a low thermally responsive rod within a highthermally responsive tube forming a probe unit to be positioned within awater heater. A free end of the element carries a movable contact whichcooperates with a fixed contact mounted on a tube which surrounds theother end of the rod and is insulated from the probe tube.

This invention relates in general to thermal switches for use in storagewater heaters and in particular to helical thermal switches in which abimetal element coils and uncoils in response to temperature variationsto make and break electrical contacts.

In automatic storage water heaters, it is common practice to provide anover-temperature switch which shuts olf the fuel supply to the waterheater when the usual thermostatic valve or valve control fails. In thepast, these over-temperature switches were usually located in the topzone of the water heater and required special wiring to the main controlunit. In addition, these over-temperature switches were usually placedin a blind location, since the water storage tank was usually covered byan outer housing.

A substantial improvement in water heater control systems is disclosedin United States Patent No. 2,409,387, issued Oct. 15, 1946 to Mr.William A. Ray and entitled, Control Device, wherein theover-temperature switch is positioned outside of the water heater butimmediately adjacent the normal control valve unit. Suchover-temperature switch consists of a single turn bimetal strip having alarge diameter, since precise movement of contacts carried by such abimetal element cannot be obtained without utilizing a bimetal elementof substantial length. Also, such an over-temperature switch was locatedoutside of the Water heater and was subject to ambient temperaturevariations which could affect the precise over-temperature cut-offpoint. In addition, such over-temperature switches were subject toaccidental damage and inadvertent or deliberate changes in adjustment.

According to the present invention, the foregoing disadvantages areovercome by providing a switch unit of the above character whichconsists of a helical bimetal element which has a plurality of turns toprovide an accurate and precise switching point; which is readilyadjustable; and which is constructed to fit inside of the normalthermostatic sensing element or probe wherein it is insulated fromexternal ambient temperature and heat losses, in addition to beingrelatively tamper-proof by virtue of its location.

Features of the present invention relate to the specific construction ofthe switch which enhances the noted adjustment; which provides reliableelectrical connections; and which is relatively free from variations inits switching point.

Other objects and features of the invention will become apparent and theinvention will be best understood when the specification and claims areread in conjunction with the accompanying drawings comprising FIGS. 1through 5 in which:

FIG. 1 shows a diagrammatic view of a conventional storage water heaterincorporating the inventive helical thermal switch;

FIG. 2 shows a partly sectionalized view of the helical thermal switchof the system of FIG. 1; and

FIGS. 3 to 5 show various cross-sectional and enlarged views of thethermal switch of FIG. 2.

Referring now to FIG. 1 of the drawings, a conventional storage waterheater has a gas control unit 2 mounted thereon for controlling thepassage of fuel from inlet pipe 3 to the pilot burner 4 and to the mainburner 5 in accordance with demand conditions determined jointly by thesetting of a temperature dial 6 and the temperature of the water assensed by the water heater temperature sensing probe 7. A pilotgenerator or thermocouple 8 is positioned immediately adjacent the pilotburner 4 and is subjected to the pilot burner flame. In response to heatfrom the pilot flame, the thermocouple 8 generates a voltage which iselectrically connected to a holding eletcromagnet 9 in the gas controlunit 2 over a circuit which includes normally closed contacts 13-28 ofprobe 7. Contacts 13-28 form a part of the inventive thermal switch andare adjusted to open at a predetermined temperature above the normaltemperature range of the water to release electromagnet 9 and terminateall fuel flow. This arrangement, thus, provides a safety feature in theevent of failure of the usual thermostatic control and a consequentover-temperature condition in the water heater.

The control unit 2 is of the well-known manual reset type gas controlunit and includes a gas cock having OFF, PILOT, and ON positions; athermomagnetic safety valve, a main line automatic valve, and anadjustable water temperature control element.

With the gas cock knob 14 in the OFF position, the flow of fuel isprevented between the main line 3 to either the pilot burner line 10 orthe main burner line 11.

With the knob 14 in the PILOT position and activated, by depressing theknob, for example, the pilot valve (not shown) operates and fuel flowsfrom the main line 3 to the pilot burner 4 over pilot line 10. The pilotvalve is mechanically coupled to armature 12 of electromagnet 9 and theabove described operation of the pilot valve moves armature 12 intomechanical association with the pole pieces of electromagnet 9. Whilethe knob 14 is depressed and the pilot valve manually operated, thepilot burner 4 may be ignited causing the voltage generated by thethermocouple 8 to energize electromagnet 9 and maintain armature 12 inits attracted position. Thereafter, knob 9 may be released, witharmature 12 now holding the pilot valve open as long as the pilot flameremains.

When knob 14 is thereafter advanced to the ON position withelectromagnet 9 still operated, the main line 3 is connected to the mainburner line 11 through a main gas valve (not shown). As is well knownthe main gas valve is operated by the thermal expansion portion of probe7 as determined by adjustable linkage associated with the temperaturesetting knob 6. When the water temperature is below the temperaturesetting knob 6, the main gas valve is open and the main burner 5 isignited by the pilot flame. When the Water temperature exceeds the notedtemperature setting, the main valve closes.

If for any reason, the voltage fed to electromagnet 9 fails, as byextinguishment of the pilot burner flame or the over-heating of thewater, armature 12 releases and closes the fluid path to both the pilotand main burner.

A system of the above character is disclosed in the noted United StatesPatent No. 2,409,387 and accordingly, a more detailed description of thecontrol unit 2 is not set forth herein. In the noted patent, theequivalent of 0 contacts 13-28 were located outside of the water heaternear the external portion of probe 7.

The invention herein relates to a helical thermal switch unit ratherthan a concentric switch as shown in the above noted patent, and is of aconstruction suitable for positioning it inside the confines of probe 7rather than being mounted external thereto.

Referring now to FIGS. 2-5, the inventive helical thermal switch unitwill be described.

The probe 7 or sensing element, is inserted into the water heater, isheld in position by shank 19 threadedly engaging a threaded opening inthe water heater housing 15. The control unit 2 is secured to probe 7,as by bolts 20, for example. The probe 7 consists generally of a coppertube 16 having one end threadedly secured to shank 19 and the other endsecured, as by welding, to an Invar rod 17 which is encompassed by thecopper tube 16 and which extends coaxially therewith. The free end ofrod 17 extends within the confines of control unit 2 and engages element18 of control 2. The copper tube 16 expands and contracts in response tochanges in water temperature and the non-expanding rod 17 transmits suchmovement to element 18 which activates a snap action thermostaticmechanism (not shown) which in turn controls the noted main gas valve ofcontrol unit 2. The above described structure is well known in the artand is one variation of the thermal bulb thermostatic element of thenoted United States Patent No. 2,409,387. For purposes of clarity, ithas been chosen to omit a showing of the normal sealing and insulatingmaterial and components for connecting the copper tube 16 to shank 19and for connecting shank 19 to the housing of control unit 2.

The helical thermal unit shown generally at 21 consists of an innercylinder or tube 22 having an inner diameter slightly in excess of thediameter of rod 17 to permit free movement of the rod 17 therethrough.The rearward end 24 of the inner cylinder 22 may be formed intohexagonal shape to enhance gripping thereof by an adjusting tool orhaving a screwdriver slot for adjustment functions as will be describedhereinafter. The forward end 23 of cylinder 22 is slightly expanded tomatch the inner diameter of a helical bimetal element 25 which has aportion near one end suitably secured, as by welding, to the expandedend 23 of inner cylinder 22. As seen most clearly in FIG. 5, the forwardfree end of the bimetal element 25 is uncurled sufliciently to makeelectrical and mechanical contact with the inner periphery of the coppertube 16. The remainder of the bimetal element 25 freely passes over theinner cylinder 22 and freely inside of the copper tube 16 to permitfrictionless coiling and uncoiling in response to ambient temperaturevariations. The rearward and free end 27 of the bimetal element 25includes a contact element 28 suitably secured thereto.

The thermal switch unit 21 also includes an outer cylinder or tube 29having one end terminating in a flange 3t) and the other end terminatingin :a forward projecting tab which has a contact 13 suitably securedthereto. The outer diameter of the outer cylinder 29 is less than theinner diameter of copper tube 16 in order to preclude electrical contacttherebetween.

The outer cylinder 29 is placed over an insulating cylindrical element31, preferably of Teflon which in turn is placed over the inner cylinder22, the thickness of the insulating element 31 being selected so as toprovide a press fit relationship between the outer cylinder 29 and innercylinder 22. As seen in FIGS. 3 and 4, insulating element 31 includes aplurality of elongated serrations which are deformed slightly to providethe noted press fit. The outer cylinder 29 is positioned longitudinallyon inner cylinder 22 so that contact 13 is in alignment with contact 28.

An insulating washer 32 of L-shaped cross-section is provided on outercylinder 29 to prevent its electrical contact with the copper tube 16and/ or shank 19. Washer 32 also permits easy insertion and removal ofhelical switch unit while providing an alignment function in maintaininginner cylinder 22 frictionally independent of the rod 17.

With the inner and outer cylinders 22 and 29 positioned with contacts 13and 28 in alignment, a suitable tool is placed over the hexagonalshaped, or slotted end 24 of inner cylinder 22 and cylinder 22 isrotated so that electrical contact is just established between contacts13 and 28 at the maximum temperature permissible in the water heater 1.With such an adjustment, electrical contact is maintained betweencontacts 13 and 28 as long as the noted predetermined over-temperaturecondition is not reached. FIG. 3 shows the position of contacts 13 and28 prior to the noted over-temperature condition.

When the ambient temperature exceeds the noted overtemperature limit,the helical bimetal element contracts and opens contacts 13 and 28 tothe position shown in FIG. 4. I

As seen in FIG. 1, contact 28 is connected to ground potential. This isaccomplished by conductor 33 being suitably connected between thehexagonally shaped, or slotted end 24 of inner cylinder 22 and the shank19. A parallel connection also exists between the copper tube 16 and thefree end 26 of bimetal element 25.

The outer cylinder 29, carrying contact 13 is insulated from groundpotential and a conductor 34 is suitably secured thereto for seriesconnection with the electromagnet 9 winding as hereinbefore described.

From the foregoing, it can be seen that the helical thermal switch unit21 is arranged to fit inside of the copper tube 16 and over the Invarrod 17 so as not to interfere with the normal thermostatic functionsthereof. Also, the switch unit 21 is readily adjustable, yet arranged topreclude misadjustment by normal thermal conditions. Still further,switch unit 21 is readily removable for repairs, cleaning and/ orreplacement. By virtue of being physically located within the confinesof the normal sensing probe 7, an over-temperature condition can besensed by the control unit 2 without external wiring and the switch isphysically located to prevent unaulhorized adjustment or tampering.

While I have described my invention in conjunction with specificapparatus and components, it is to be understood that numerous otherembodiments are possible without departing from the spirit of theinvention.

I claim:

1. In a thermal switch including a tube-like body having a high thermalcoeflicient of expansion, a support for said body, and a rod having oneend fixed to one end of said body, said rod having a cross sectionsmaller than that of said body, said rod extending through said body inspaced relation to the internal surface thereof, the combinationcomprising: a bimetallic helix in the said space between said rod andsaid body, said helix having an axis extending along the lengths of saidrod and said body inside of said body, said helix being spaced both fromsaid rod and said body from one end of said helix to a portion thereofnear the other end thereof, said helix including a first metal layer atits outside diameter and a second metal layer at its inside diameter,said first and second metals having different thermal coeflicients ofexpansion; first means to hold only said portion of said helix near saidother end in a fixed position in the space between said rod and saidbody, said one end of said helix being free to move relative to said rodand said body; a first electrical contact fixed to said one end of saidhelix and movable therewith; a second electrical contact; second meansto support said second contact in a fixed position to be engaged by saidfirst contact; first and second electrical leads, said first meanselectrically connecting said first lead to said portion of said helixnear said other end; and means electrically connecting said second leadto said second contact, said helix serving both as a bimetallic elementand as an electrical conductor between said first contact and saidportion of said helix near said other end; said helix being a spiraltape having a width along the length of said helix greater than thethickness thereof, said body being a cylindrical tube, said rod alsobeing cylindrical, said tube, said rod and said helix all having acommon longitudinal axis, said helix having at least a portion of an endturn of an increasing diameter at the said other end thereof, theunstressed outside diameter of said end turn portion being larger thanthe inside diameter of said body, said end turn thereby lying inpressure contact with said body, said first means including acylindrical conductive tube having at least a portion contiguous to butrotatable around said rod, said first means tube extending through saidhelix from said one end to said portion thereof in spaced relationthereto, said helix portion being welded to said tube, said second meansincluding a first cylindrical insulating tube contiguous to andsurrounding said first means tube, a conductive tube contiguous to andsurrounding said first insulating tube, and an annular insulatorcontiguous to and surrounding said second means conductive tube, saidsecond means also including means to hold said annular insulator in afixed position relative to said support, said second means conductivetube having a projection to carry said second contact.

2. In a thermally responsive control device including an elongated maintube of high thermal expansive characteristics adapted to be mounted ina support adjacent the wall of a chamber to be heated, with the forwardend of the tube extending into the chamber, and an elongated rod of lowthermal expansive characteristics positioned within the tube with itsforward end attached to the forward end of the tube, and the remainderof the rod spaced from the surrounding tube, an elongated temperatureresponsive switch unit positioned within the space between the tube andthe rod, the unit comprising: an elongated conductive inner tubesurrounding a portion of the rod extending through the wall of thechamber and surrounding a major portion of the rod extending into thechamber, the inner tube having a considerable portion of its lengthcontiguous to but movable with respect to the rod; an elongatedbimetallic helical element surrounding a forward portion of the innertube with a forward portion of the element being attached to a forwardportion of the inner tube, and with the portions of the elementextending rearwardly from the attachment point loosely surrounding theinner tube, a contact mounted on the rear end of the helical element andthe bimetallic element being formed such that the contact is arcuatelymovable in response to ambient temperature changes; an elongated outerconductive tube surrounding the rear portion of the inner tube andcarrying on its forward end a fixed contact which cooperates with themovable contact on the helical element, the outer tube being insulatedfrom the main tube; and a tubular insulator extending snugly between theinner end outer tubes.

3. The control device of claim 2 wherein the outer tube has its forwardend terminating in a forwardly extending projection carrying the fixedcontact and the rear end of the tube terminates in an outwardlyextending flange; and including an insulator with an L-shapedcrosssection surrounding the rear end of the outer tube for insulatingthe outer tube with respect to the main tube and for properly aligningthe inner and outer tubes within the main tube.

4. The control device of claim 2 wherein the forward end of the innertube has an enlarged diameter with respect to the remaining portion ofthe inner tube to facilitate attachment to the bimetallic element; andthe forward turn of the helical element has an outside diameter whichwhen unstressed is larger than the inside diameter of the main tube suchthat the forward turn is pressed against the main tube.

5. The control device of claim 2 wherein the frictional fit between thetubular insulator and the inner and outer tubes is such that the angularorientation of the inner and outer tubes may be varied but the inner andouter tubes will remain in the orientation selected due to thefrictional engagement with the tubular insulator thereby permittingangular adjustment of the contacts relative to each other.

References Cited UNITED STATES PATENTS 1,574,508 2/1926 OKeefe.

3,291,390 12/1966 Solomon 200-137 X 3,093,887 6/1963 Prestige et a1.

2,575,481 11/1951 Anderson -19 2,321,852 6/1943 Ray 200138 BERNARD A.GILHEANY, Primary Examiner. H. A. LEWITTER, R. COHRS, AssistantExaminers.

